1. Field of the Invention
The present invention pertains to a novel high performance charge-coupled device (CCD) image sensor for use in optical imaging systems and particularly to a new device design and architecture providing enhanced sensitivity and control of charge carriers concurrent with higher pixel density and smaller sensor size.
2. Description of Prior Art
The key problem existing in all solid-state image sensors, in addition to the requirement for efficient sensing and conversion of an image into pixels of charge representing an image signal, is addressing and delivering the image signal to the sensor output terminals with high speed and in a predetermined sequence. The CCD approach has been popular in its use for this purpose because of its inherent ease in manipulation and transport of charge within a silicon chip. However, the disadvantage of the CCD approach resides in its requirement for wide charge transfer channels which consume valuable image sensing area and in the sensitivity to collection of stray charge which leads to image smear. As the demand for image sensors having high resolution, high pixel density and small size steadily grows, several techniques have been developed for enhancement of CCD image sensors to minimize these limitations on sensor performance.
One such enhancement technique has combined a charge sweep method with interline transfer (IT) architecture commonly used with CCD image sensors. ("Super Small TV Camera Using CSD Device," K. Takashima, Image Technology and Information Display, Vol. 20, No. 9, pp. 47-51, May 1988). This technique significantly decreases the size of the vertical charge transfer channels interposed between columns of photosite sensing elements and thus improves the quantum efficiency of the CCD. However, the time required for charge sweep from the sensing element in the array is relatively large and the limitation of unwanted smear is not completely resolved.
The present invention describes a new CCD image sensor and methodology which overcomes the limitations of conventional CCD image sensors and which provides high resolution, high pixel density, decreased size and greatly enhanced performance. New sensor architectures are described which use a charge super sweep (CSS) technique to overcome common CCD problems such as blooming and image smear. The described invention also provides image sensors having intrinsic exposure control, using an electronic focal plane shutter technique, with a single or variable scan rate.
The charge super sweep technique allows the fast transfer of several lines of data from photosites located anywhere in a pixel array into a buffer storage area located at a lower edge of the image sensing region of the sensor. The transfer is performed during every horizontal blanking interval, or only during the vertical blanking interval if the transfer of a full frame of data is desired. For the image sensor having frame transfer capability, one or more on-chip analog field memories are incorporated similar to conventional frame transfer CCD sensors and the sensor can be used for still picture photography applications.
The charge super sweep (CSS) technique uses very narrow vertical channels located between the photosites similar to interline transfer (IT) devices. The CSS technique is distinguishable, however, in that the charge from the photosites is never stored in the channels for any significant length of time and is swept out using a resistive gate traveling wave technique. The narrow channels minimize loss of light sensitivity and the traveling wave sweep maximizes the charge transfer efficiency in the narrow channels by using a potential field enhanced charge carrier drift effect.